1. Field of the Invention
The present invention relates to a method for producing laminated iron cores which are formed of fixedly stacked stator sheet pieces and rotor sheet pieces obtained by blanking a strip material, and also to the resultant laminated iron cores.
2. Description of the Related Art
For example, such a laminated iron core constituting a stepping motor as a stator S or a rotor R shown in FIG. 22 is produced by sequentially stacking iron core sheet pieces (stator or rotor sheet pieces) of a predetermined shape obtained by blanking a strip material and then by bonding the sheet pieces to each other.
When it is desired to produce a laminated iron core such as the aforementioned stator S or rotor R, there is considered such a method as to blank out the rotor sheet piece by utilizing an inner diameter zone of the stator sheet piece where a hole for accommodating the rotor is formed.
As shown in FIG. 23, however, an air gap a defined between the inner periphery of the stator S and the outer periphery of the rotor R is set to be as extremely small as about 0.02 mm, so that, after the rotor sheet piece is blanked out from the inner diameter zone of the stator sheet piece, a blanking width only as small as the air gap a is left in the inner peripheral edge portion of a semi-stator sheet piece which will be later formed into the stator sheet piece of a predetermined shape.
As a result, when the above semi-stator sheet piece is subjected to a blanking operation at its inner peripheral edge portion to make the above rotor accommodation hole of the predetermine shape, the blanked scrap slug must have a narrow ring shape corresponding to the air gap. However, because of too narrow width, the scrap ring tends to be broken at its several portions and the broken scums or slugs tend to be projected on the surface of the material without falling into a lower punching die, that is, a so-called "scum riser" phenomenon takes place, which disadvantageously results in a faulty product having dimples or the like.
For the purpose of solving such a disadvantage, there has been conventionally employed such a method that rotor and stator sheet pieces are made by blanking respectively different strip materials with use of respectively different dies. In this method, however, in addition to a fact that two die machines and press machines are required for producing the stator and rotor for one type of stepping motor, the yield of the material is also poor which results in production costs being inevitably increased.
Meanwhile, as a conventional technique for overcoming the above-mentioned disadvantage that the blanking gap is narrow, there is suggested a method as disclosed, e.g., in Japanese Patent Application Laid-open No. 1-190235, in which before blanking out the rotor sheet piece from a metal thin plate, a bead-like drawing portion is formed at a location of the metal thin plate having a diameter larger than the diameter of the rotor sheet piece, which drawing portion is flattened to be extended toward the blanked side of the rotor sheet piece after the blanking of the rotor sheet piece, and thereafter, the stator sheet piece is blanked out.
With the above structure, since the metal thin plate is made flat to be extended prior to the blanking of the stator sheet piece, the gap allowance or blanked scrap width can be secured, whereby a scrap piece can fall in each blanking stroke without being broken during the blanking operation, and thus, a twice blanking operation problem can be solved.
However, in the above arrangement, since the drawing portion is made flat by pressing it, there remains wavy. undulations on the metal thin plate after subjected to the pressing operation. As a result, there will also remain undulations on the stator sheet piece obtained by blanking the thin plate having the undulations. Thus, a resultant iron core of such laminated stator sheet pieces exhibits an improper shape with an undesirable skew angle. That is, the resultant iron core has a disadvantage that precision in the squareness and parallelism is reduced.